In electric discharge machining apparatus, automatic pallet changers may be used for loading and unloading a workpiece.
FIG. 7 illustrates a conventional electric discharge machining apparatus and an automatic pallet changer 80. The basic components of the electric discharge machining apparatus comprise a machine body 70 and a power generator 60 for supplying voltage pulses. The machine body 70 includes a bed 71 on which a cross table (not illustrated) is provided. A work tank 56 is overlaid on the cross table and is movable in a horizontal plane defined by orthogonally disposed X and Y axes. In the work tank 56, a positioning jig 3 and a leveling pin 4 are fixed on a work table (not illustrated). The machine body 70 further includes a column 72 which, in the illustrated embodiment, is provided rearwardly of the bed 71. A head part 74 is supported on the column 72. A tool electrode 31 is attached to the head part 74 by means of a tool holder 75. In the head part 74, a motor is provided for moving the tool electrode 31 in a Z axis direction which is orthogonal to the horizontal plane formed by the X and Y axes.
The pallet changer 80 includes a post 81 which can be raised and lowered. A turntable 82 is supported on the post 81. An arm driver 83 is provided on the turntable 82 for extending and retracting an arm 84.
When preparing to machine, the pallet changer 80 places a pallet 1, on which a workpiece 14 is clamped (by means of clamps), on the arm 83. The arm 83 is positioned to oppose the work tank 56 by rotating the turntable 82. The pallet 1 may be precisely placed on the positioning jig 3 in the work tank 56 by extending, lowering and then retracting the arm 83. Then, as shown in FIG. 6, after raising the side walls 26 of the work tank 56, the work tank 56 is filled with dielectric fluid 55.
During the EDM process, electric discharges are induced in a machining gap 32 formed between the tool electrode 31 and the workpiece 14 by applying voltage pulses thereacross. The dielectric fluid 55 in the machining gap 32 is heated by the discharge current, thereby producing dissociated gases. Accordingly, there is a need for cleaning and cooling the machining gap 32, such as by flushing or suctioning the dielectric fluid through holes provided in the workpiece 14 and/or the tool electrode 31, to prevent the machined particles and the gases from accumulating in the machining gap 32.
Further, a flushing attachment 57 may be provided on the work table 2, using, for example, a magnetic clamp 59, as shown in FIG. 6. The flushing attachment 57 includes a piping block 27 having a passage 58 therein. A flexible hose 30, which may be bent into a desired orientation and is preferably equipped with a nozzle 29, extends upwardly from the piping block 27. If necessary, a plurality of passage 58 may be provided in the piping block 27 so that a plurality of flexible hoses 30 can be used. The passage 58 is connected to a dielectric fluid supply tank through a pipe 21 and a coupler 24 attached to the work table 2.
However, in the arrangement shown in FIG. 6, it is necessary to retract the flexible hose or hoses 30 when loading or unloading the workpiece 14. In addition, manual bending of the flexible hose or hoses 30 into a desired form is again required after the pallet 1 is placed on the positioning jig 3. Thus, even though automation in the EDM process may be realized by a NC unit, it is still not possible to perform completely unmanned loading of the workpiece.